Conventionally, MOS transistors are formed in an active zone of a semiconductor substrate that is isolated from the rest of the substrate by an insulation region. The insulation region may be, for example, a region of the type with shallow trenches (STI: shallow trench isolation). Source and drain regions are formed which delimit a channel between them, and a gate region is formed above the channel. All of this is covered with a dielectric layer in which contact holes are etched for electrical connection of the transistors. An etch-stop layer, for example, a nitride layer without any edges, is between the substrate and the dielectric layer.
Residual stress of the etch-stop layer modifies the performance of the transistor, in particular, the current ION flowing between the drain and the source in the on-state. This current may be increased or decreased depending on the level of the mechanical constraint applied to the transistors. This is due to the fact that the stop layer induces a local curvature in the substrate of the semiconductor device, which generates a mechanical constraint in the channel. Such a constraint effects the mobility of the carriers, and therefore the performance of the transistor.
By using these properties, the performance of the transistors can be improved by modifying the residual stress level of the stop layer through the implantation of germanium (Ge) in a stop layer that includes nitride. This can be done since the insertion of ions into the nitride destroys the mechanical constraint applied to the silicon.
An etch-stop layer having a negative residual stress level improves the operation of a PMOS transistor but degrades the operation of an NMOS transistor. Conversely, an etch-stop layer having a positive residual stress level improves the operation of NMOS transistors but degrades that of PMOS transistors. Hence, any improvement made to one type of transistor is obtained at the detriment of the other type of transistor. Consequently, ion implantation in the nitride makes it possible either to improve the operation of PMOS transistors or to improve the operation of NMOS transistors, depending on the type of stop layer used.